Panic when using fuzzy completion in a folder containing files starting with a multibyte unicode char

[dburburan]

Describe the bug

When fuzzy matching is on, and you have files starting with multibyte chars, and you type some letters and hit tab, you can trigger a panic.

This is related to a number of existing issues but I think I found a distinct case.
nushell/nushell#15302
nushell/nushell#13951
nushell/nushell#12680
nushell/nushell#11916

The bug is in the reedline crate, which I had a look at, but I'm putting the issue here as I think this where most of the issues are logged.

There were a couple of recent PRs that fixed some of these unicode issues -
#903
#886

But they don't fix this one, it still panics.

How to reproduce

You need fuzzy completions enabled in config.nu:

$env.config.completions.algorithm = "fuzzy"

And then do this

mkdir testdir
cd testdir
touch "abcdef"
touch "验abcdef"
ls ac<tab>

You get the following, or maybe it just drops out of completion, sometimes the panic message is suppressed.

x Main thread panicked.
  |-> at ~\reedline\src\menu\columnar_menu.rs:318:46
  `-> byte index 2 is not a char boundary; it is inside '验' (bytes 0..3) of `验abcdef`

Note that ab<tab> will not crash, only ac<tab>
This is because ab is a substring of the filename, but ac is not, and only produces matches in fuzzy mode.

It happens because in reedline/src/menu/columnar_menu.rs:311 (fn create_string) it tries to search for an exact substring to underline, but in this case it finds nothing, so match_position defaults to 0.

            let match_position = suggestion
                .value
                .to_lowercase()
                .find(&shortest_base.to_lowercase())
                .unwrap_or(0);

When the next line tries to slice the match result, it's assuming that we found the substring. If we did, then match_len would be the correct byte length and the slice would work properly.

In this case though it tries to slice 2 bytes from position 0 (we typed ac so that's length 2) but the first character of our file is 验 which is actually 3 bytes long, so the slice is mid-char and panics.

            let match_str = &suggestion.value[match_position
                ..match_position + match_len.min(suggestion.value.len() - match_position)];

Expected behavior

Don't panic

Configuration

I built a recent version to check it was still happening

key	value
version	0.105.1
major	0
minor	105
patch	1
branch	main
commit_hash	1fe62ee6132372453d6bfeb95978ee61a2b77f3b
build_os	windows-x86_64
build_target	x86_64-pc-windows-msvc
rust_version	rustc 1.85.1 (4eb161250 2025-03-15)
rust_channel	1.85.1-x86_64-pc-windows-msvc
cargo_version	cargo 1.85.1 (d73d2caf9 2024-12-31)
build_time	2025-06-11 06:54:27 +10:00
build_rust_channel	debug
allocator	standard
features	default, sqlite, trash
installed_plugins

[ysthakur]

Thanks for putting so much effort into reporting this. I moved this to the Reedline repo since it affects all Reedline consumers, not just Nushell.

This is a known issue and I had a PR up for it (#887), but I forgot about it, sorry. Anyway, that old PR is overly complicated.

[dburburan]

Ahhh I missed that PR. Thanks for all your efforts, it's a nice project.